Javascript Formula to convert WGS84 lat, long values to RD_New co-ordinates

Question

I have a requirement in my project to convert WDS84 GPS format co-ordinates to RD_New co-ordinates

WGS84(E,N): 51.998929,4.375587
RD(X,Y):    85530,446100

I see some online tools and Proj4 js libraries to so this conversion. But i need some Formula so i can directly convert values in my Angular project.

Below link converts from RD_New to WDS84. http://www.aukema.org/2020/02/convert-dutch-rd-coordinates-to-wgs-84.html

I couldn't find Formula for RD_New conversion